[PATCH] block, nvme: export and use passthrough stats

[Christoph Hellwig]

On Fri, May 22, 2026 at 08:15:37AM -0700, Keith Busch wrote:
> From: Keith Busch <kbusch at kernel.org>
> 
> So stacking drivers can also report passthrough workloads through
> iostat.

Except that this patch also wires it up in nvme.  So please split
it and write proper commit messages for both parts.

> +static inline bool blk_rq_passthrough_stats(struct request *req)
> +{

And maybe add a kerneldoc comment now that this is public.